Misner Thorne and Wheeler [General Relativity] was the textbook we used back when I took intro AP. The course was heavy on gravity and GR. It was a second year course of comparable difficulty to a second year physics course [intro to quantum mechanics and statistical physics].

I am currently taking an introductory course in astrophysics and I love it. Just as Chronos says it demands a good understanding of undergraduate mathematical courses i.e. Calculus and basic linear algebra. It also expects a deep understanding of newtoninan mechanics and it is to your advantage if you have read some special relativity as well.

But how the courses are constructed may vary a lot. Therefore i would advise you to read the course-descrption carefully so that you know what you are expected to understand and do during the course.

If you like mathematics and physics or just science in general you will probably find the course very interesting. Even so, having a lot of the first two is probably necessary.